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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

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142                                  ALASKAN GLACIER STUDIES
and swept powerfully westward through the fiord. The ice of Cascading Glacier pours over the lip of its valley down to a point just below the second pronounced rock bench (PLLXIH), where its further advance is checked, partly by ablation and partly by the discharge of fragments that tumble down from its terminus. The melting of the glacier has produced no single large stream, but a multitude of small rills extend from its front down to the (Jiff base. These tiny streams have not sufficient volume, and have not been at work for a long enough time to have erased the glacial grooves produced by the formerly-expanded, westward-moving, Nunatak Glacier. The largest stream, itself of small size, emerges from the west end where the ice rests on the upper rock bench. Apparently the downward extension of Cascade Glacier is mainly checked by the falling of ice blocks rather than by ablation. During each of our visits to this glacier we either heard or saw the fall of the ice blocks at frequent intervals. Usually these falls consist of only small masses, but oftentimes great blocks were detached. There is an extensive talus of these fallen ice blocks, mixed with rock fragments, rising almost up to the terminus of the glacier.
By these falls of ice the form of the lower edge of the Cascading.Glacier has been materially altered since it was first observed by Gilbert in 1899, the greatest part of the change having occurred between. 1899 and 1905, though there have been noticeable changes between 1905 and 1909. In 1899 a broad tongue of ice extended from the lower bench down the steep cliff face to a moraine area at its base, which was probably the lateral moraine of the land tongue of Nunatak Glacier. This projecting point of Cascading Glacier had almost entirely disappeared in 1909 and the total recession at this point has amounted to between 200 and 300 feet. Its form changed slightly between 1909 and 1913. There have been minor changes in other parts of the Cascading Glacier margin, but nothing noteworthy excepting the developing of a moraine-covered area on the west side just above the lower bench.
There is no sign of response of this glacier to advance through earthquake shaking, and in view of the fact that it has been examined and photographed in 1899, 1905, 1909, 1910 and 1918, each of the latter four years showing recession, we feel warranted in assuming that this glacier has not made any advance whose effect extended beyond the lip of the mountain valley. In view of the shortness of the glacier we assume that it will not in the future respond to, the influence of earthquake shaking. Why this glacier should not respond, as other glaciers have done, is not easy to understand, unless it is due to the fact that in this high-lying valley, in the midst of mountains which do not rise to any great height above it, there are not extensive slopes on which the snow rests with sufficient instability to be shaken into the valley in great enough quantities to cause an advance in the glacier.
Significance of the Cascading Condition. Cascading Glacier is one of many glaciers-of the same type, found not only in the Yakutat Bay region, but in many other parts of the glacial region of Alaska. Indeed, the name has suggested itself to others besides Gilbert, and there are already at least three other glaciers of this name in Alaska, beside glaciers with names like Cataract Glacier, Hanging Glacier and Toboggan Glacier, which are evidently of the same type. This cascading condition seems to be dependent upon two factors widespread in Alaska; first, the former extension of great trunk glaciers which have so lowered the main valleys by glacial erosion as to leave the tributaries hanging high above their bottoms; and, secondly, the uncovering of the steepened slopes below the-